A stretching frame can comprise a pair of rails extending along edges of the path of a web to be stretched and can have mutually divergent portions in the stretching plane so that the gripper carriages as they are displaced along the rail and which can engage opposite edges of the web, travel away from one another to effect traverse stretching while applying tension to the web and generate longitudinal stretching thereof. The result is a bidimensional or biaxial stretching of the web.
In German Patent Application DE-AS 2 058 575 equivalent to U.S. Pat. No. 3,748,704 the latter being herein incorporated by reference, a stretching frame assembly for such purposes is described. This assembly can have a one-piece guide rail having running surfaces parallel to the stretching frame plane and defining a U-shaped guide channel between shanks of which respective guide rollers of the gripper carriage are received. A drawback of this system is that these guide rollers uncontrollably engage alternately one guide surface and the other so that the direction of rotation of the guide rolls respectively changes depending upon which surface is engaged. This has been found to have undesirable effects upon the manner in which the carriage moves, the loading of the guide surfaces and the like.
The gripper carriage in this arrangement has coupling means for various transport devices, namely, a roller for a guide track and a pin for a transport chain. However, the different transport devices act upon the carriage differently from one another. A shock free engagement by a transport device or shock free transition between the disengagement of one transport device and the engagement of another as the carriage travels along the rail is therefore difficult to achieve. This also applies to transport devices having adjustable travel speed.
By and large, therefore, it can be said that the stretching frame assemblies described in the aforementioned reference can be improved upon.